Local electrochemical characteristics

In the paper from V. Matveyev of the Ukrainian State University of Chemical Engineering, an examination of the role of conductive carbon additives in a composite porous electrode is conducted. A model for calculation of the local electrochemical characteristics of an electrode is presented. A comparison on the polarization of the electrode as a function of the redox state of the electroactive species is emphasized in the model. The electrochemical reaction of chloranil (tetrachlorobenzoquinone) was measured and results compare favorably to calculations derived from the model. 

The model for calculation of the local electrochemical characteristics of an electrode is presented below. 

Subcell Approach Stumper et al.135 presented the subcell approach to measure localized currents and localized electrochemical activity in a fuel cell. In this method a number of subcells were situated in different locations along the cell s active area and each subcell was electrically isolated from each other and from the main cell. Separate load banks controlled each subcell. Figure 8 shows the subcells in both the cathode and anode flow field plates (the MEA also had such subcells). The current-voltage characteristics for the... 

Maepherson, J.V., de Mussy, J.P.G., and Delplancke, J.L., Conducting-atomic force microscopy investigation of the local electrical characteristics of a Ti/TiOj/Pt anode, Electrochem. Solid St. 4, E33-E36, 2001. 

SECM (Scanning electrochemical microscopy) is a technique to characterize the local electrochemical nature of various materials by scanning a probe microelectrode [1,2]. The spatial resolution of SECM is inferior to the conventional scanning probe microscopes such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) as the fabrication of the probe, microelectrode, with nanometer sizes is quite difficult and the faradaic current of the microprobe is very small (often picoamps or less). However, SECM has unique characteristics that cannot be expected for STM and AFM SECM can image localized chemical reactions and it also can induce localized chemical reactions in a controlled manner. 

Thiedmann, R., Hartnig, C., Manke, 1., Schmidt, V., and Lehnert, W. (2009) Local structural characteristics of pore space in GDLs of PEM fuel cells based on geometric 3D graphs. J. Electrochem. Soc., 156, B1339-B1347. 

The alloy composition (and microstructure) has strong effects on all the aspects of passivity that have been described above chemical composition and thickness of the passive film, electronic properties, structure, and kinetics of formation. The influence of alloyed elements on the electrochemical characteristics of passive systems can be seen in Fig. 3-16. This is the same current-potential curve as in Fig. 3-1, on which the two major effects of alloyed elements are indicated lowering of the dissolution current in the active region and at the active-passive transition, and broadening of the passive region. A third effect, not illustrated in Fig. 3-16 but which will be discussed later, is the improvement of the resistance of the alloy to passivity breakdown and localized corrosion. For iron-based alloys, these beneficial effects are obtained with chromium, molybdenum, nickel, and nitrogen. 

With the development of fuel cell technology, many different investigative tools, including electrochemical and physical/chemical methods, have become available that elucidate CL degradation. These methods provide valuable information on morphology (surface or cross-section of the CL, size distribution of the catalyst particles), elemental content and distribution, atomic structure of the local particles inside the CL, and electrochemical characteristics of the CL in fuel cell systems. At present, characterization of PEM fuel cell electrodes mainly concentrates on morphological characteristics (surface and microstructure), electrochemical diagnosis, and composition analysis. 

Aluminum alloy microstructures are developed as a result of alloy composition and thermomechanical treatment. From a corrosion perspective, the dominant features of alloy microstructure are grain structure and the distribution of second phase (intermetallic) particles as constituent particles, dispersoids, or precipitates. Such particles have electrochemical characteristics that differ from the behavior of the surrounding alloy matrix, making alloys susceptible to localized forms of corrosion attack that has been termed microgalvanic corrosion. 

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

Esr spectroscopy has proved useful for characterizing the reduced species, enabling assessment of whether the unpaired electron density is localized on the metal or delocalized onto the ligand. A typical study is the reported electrochemical reduction of the Ni(n) complex of (289) (Bailey, Bereman, Rillema Nowak, 1984) a reversible one-electron reduction occurs to yield a product whose esr spectrum shows two g values which are characteristic of a Ni(i) derivative. In contrast, the reduction product formally represented by (290) has been shown to have extensive delocalization onto the ligand it is probably best described as involving coordination of afree radical to a central Ni(n) (Lovecchio, Gore Busch, 1974). 

The speed of p- and n-type doping and that of p-n junction formation depend on the ionic conductivity of the solid electrolyte. Because of the generally nonpolar characteristics of luminescent polymers like PPV, and the polar characteristics of solid electrolytes, the two components within the electroactive layer will phase separate. Thus, the speed of the electrochemical doping and the local densities of electrochemically generated p- and n-type carriers will depend on the diffusion of the counterions from the electrolyte into the luminescent semiconducting polymer. As a result, the response time and the characteristic performance of the LEC device will highly depend on the ionic conductivity of the solid electrolyte and the morphology and microstructure of the composite. 

The properties characteristic for electrochemical nonlinear phenomena are determined by the electrical properties of electrochemical systems, most importantly the potential drop across the electrochemical double layer at the working electrode (WE). Compared to the characteristic length scales of the patterns that develop, the extension of the double layer perpendicular to the electrode can be ignored.2 The potential drop across the double layer can therefore be lumped into one variable, DL, and the temporal evolution law of DL at every position r along the (in general two-dimensional) electrode electrolyte interface is the central equation of any electrochemical model describing pattern formation.3 It results from a local charge bal-... 

A characteristic feature of the carbon modifications obtained by the method developed by us is their fractal structure (Fig. 1), which manifests itself by various geometric forms. In the electrochemical cell used by us, the initiation of the benzene dehydrogenation and polycondensation process is associated with the occurrence of short local discharges at the metal electrode surface. Further development of the chain process may take place spontaneously or accompanied with individual discharges of different duration and intensity, or in arc breakdown mode. The conduction channels that appear in the dielectric medium may be due to the formation of various percolation carbon clusters. 

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